1. Field of the Invention
The invention concerns an electromagnetic detector and, more particularly, a wave detector with quantum well semiconductors.
2. Description of the Prior Art
There are known radiation detection devices including at least one compositionally asymmetrical quantum well formed by a stack of materials with different forbidden gaps.
Thus, the French patent application No. 88 15956 filed on Dec. 6, 1988 describes a structure designed for the photocapacitive detection and the processing of optical radiation. This structure is formed by asymmetrical quantum wells as shown in FIG. 1a. During an illumination, the electrons are excited from an electron level e1 to an electron level e2, as the two levels do not have the same barycenter. This transition induces a dipole that can be detected at the terminals of a capacitor formed by such quantum wells. Other non-linear effects (such as frequency doubling or electro-optical effects), which may be used in modulators etc., are associated. Such effects have been confirmed experimentally (see Electronics Letters, E. Rosencher et al., "Second Harmonic Generation by Intersub-band Transitions in Compositionally Asymmetrical MQWs" Vol. 25, 16, Aug. 3, 1989, page 1063).
The invention concerns a structure that reinforces all these different above-mentioned effects. In order to highlight the object of the invention more clearly, let us return to the structure of FIG. 1. Under illumination, the rate of generation from the electron level e1 to the electron level e2 is (FIG. 1): EQU G.sub.12 =B.sub.12 (n.sub.1 -n.sub.2).phi.
.phi. is the photon flux, PA1 n.sub.1 and n.sub.2 are the populations at the levels e1 and e2; PA1 B.sub.12 is the coefficient of induced emission called EINSTEIN's coefficient. PA1 .delta..sub.z is the mean displacement of the electron gas PA1 q is the charge of the electron. PA1 the optical transition e.sub.A towards e.sub.B, PA1 and hence by the measurement of the variation in conductance of the structure due to the differences in electron mobility in each well. PA1 the energy corresponding to a second electron level is between the potential energy of the bottom of the conduction band of the material of the fourth layer and the potential energies of the bottom of the conduction band of the materials of the second and sixth layers, PA1 the energy corresponding to a third electron level is between the potential energy of the bottom of the conduction band of the material of the fourth layer and the potential energy of the bottom of the conduction band of the material of the fifth layer, this third electron layer being greater than the first electron level;
The recombination is given by: EQU R=n.sub.2 /.tau.
where .tau.) is the time of relaxation from the level e2 towards the level e1. The polarization induced by the illumination is: EQU P=n.sub.2 q.delta.z
that is, EQU P=q.delta..sub.z .tau..sub.12 B.sub.12 (n.sub.1 -n.sub.2).phi.
where:
we therefore have =n.sub.2 =.tau.B.sub.12 (n.sub.1 -n.sub.2)0 It is clear that by increasing the lifetime .tau., we increase the induced dipole.
There is also a known device having a conduction band curve such as the one described in the U.S. Pat. No. 4,745,452 shown in FIG. 1b.
This device has a quantum well formed by two coupled quantum wells QWA and QWB. Two permitted electron levels e.sub.A and e.sub.B are below an intermediate barrier BA. However, there is a strong coupling between the levels e.sub.A and e.sub.B owing, notably, to the small thickness of the intermediate barrier BA (thickness smaller than 20 nm for example). The detection is then done by:
A device such as this is limited to the detection of the electromagnetic waves below some TeraHz, that is, some meV.
The invention relates to a device working with waves that attain 30 to 100 TeraHs, that is, about 100 meV.
The invention concerns a detector enabling this problem to be resolved.